Field of the Invention
The present invention relates to a machine tool.
Discussion of the Background
A thermal expansion of each of members constituting a machine tool due to heat generated by operating the machine tool and heat from the surroundings of the machine tool is a significant cause that brings the relationship in position between a blade edge of a tool and a machining workpiece to a misaligned state. As a result, the machining accuracy may be deteriorated. Such a misalignment due to heat in the relationship in position between the blade edge of the tool and the machining workpiece will be referred to as a thermal displacement hereinafter. In general, an NC device for controlling the machine tool includes the function of correcting the thermal displacement. In this thermal displacement correction function, a method that allows each of temperature sensors to be attached to a corresponding one of portions of major members constituting the machine tool, and that allows temperature values each resulting from multiplying a temperature value at a corresponding one of the portions by a coefficient to be summed to obtain a thermal displacement correction amount is generally employed. There are, however, a large number of and various heat sources each likely to be a cause of the thermal displacement, and further, for each of all members subjected to the influence of heat, the degree and a direction a thermal expansion of the each member are needed to be taken into consideration. Further, it is extremely difficult to accurately analyze all causes and make thermal corrections with high accuracy. In particular, a temperature variation due to external causes existing outside the machine tool is unanalyzable in principle, and thus, the thermal displacement is extremely difficult to correct with accuracy.
In a thermal displacement correction control device disclosed in Japanese Patent No. 5490304, the position of a reference ball disposed at the outside of a machining area is measured at intervals of a predetermined period of time to obtain a displacement amount of the ball and record the displacement amount together with temperatures of a plurality of portions. With respect to the value of a coefficient for use in a calculation formula for estimating a thermal displacement amount using the temperatures of the plurality of portions, an equation for obtaining the coefficient is created by substituting a plurality of the recorded displacement amounts and a plurality of the recorded temperatures into the estimation calculation formula to solve the equation and obtain an optimum value for the coefficient. Further, displacement amounts resulting from measuring the reference position, displacement amounts of the reference position resulting from correcting the displacement amounts using a currently effective thermal displacement correction coefficient, and displacement amounts of the reference position resulting from applying a thermal displacement correction coefficient having been obtained through the above calculation are each displayed in the form of a graph to allow a worker to select acceptance or rejection in relation to the employment of the thermal displacement correction coefficient, which has been obtained through the above calculation.
In a device disclosed in Japanese Unexamined Patent Application Publication No. 60-228055, machining sizes during a period of time that is immediately after the start of a machining operation and that is when the thermal status has been stable are recorded together with clock times, and afterward, when a machining operation is performed on the same workpiece, a thermal correction is made on the basis of a recorded period of time from the start of the machining operation and a recorded machining size at that time point. This configuration enables obtaining satisfactory machining sizes even in a thermally unstable state immediately after the start of the machining operation.
Further, in a device disclosed in Japanese Unexamined Patent Application Publication No. 2006-116663, a spindle speed and a spindle load are detected, and through the use of a calculation formula based on a previously estimated thermal displacement, an actual thermal displacement amount is estimated to correct the thermal displacement.